Hydraulic circuit for traveling

ABSTRACT

If a hydraulic motor ( 23 ) (traveling wheel ( 27 )) is rotated (or skidding) at high rate, and a hydraulic motor ( 24 ) is hardly rotated, a detection controller ( 68 ) compares the rotational speeds of both hydraulic motors ( 23, 24 ). Then the rotation detected by the rotation detectors ( 66, 67 ) to detect the traveling wheel ( 27 ) to be skidding, and moves a piston ( 71 ) to allow a negative brake ( 40 ) to give a braking force to the hydraulic motor ( 23 ), and rotate the hydraulic motor ( 24 ). In this way, this invention can release the traveling wheel from skidding without the use of a flow dividing valve.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a hydraulic circuit fortraveling in which two hydraulic motors for driving the traveling wheelscan be rotated by employing one hydraulic pump.

[0002] A related hydraulic circuit for traveling in which two hydraulicmotors for driving the traveling wheels are rotated by one hydraulicpump is, for example, shown in FIG. 3.

[0003] This hydraulic circuit comprises: a hydraulic pump 11; twohydraulic motors 14, 15 for driving and rotating the traveling wheels12, 13, respectively; a pumping passage 16; a flow dividing valve 17;and an opening-closing valve 19. The pumping passage 16 comprises oneend connected to the hydraulic motor pump 11 and is bifurcated halfwayto provide other ends respectively connected to the hydraulic motors 14,15 so as to supply a high pressure fluid discharged from the hydraulicpump 11 to the two hydraulic motors 14, 15. The flow dividing valve 17serving to supply an equal amount of fluid to the hydraulic motors 14,15 is provided at the branch portion of the pumping passage 16. Theopening-closing valve 19 is interposed halfway in a connection passage18 for connecting the bifurcate portions 16 a, 16 b in the pumpingpassage 16.

[0004] And in this hydraulic circuit, if any one of the traveling wheel,for example, a traveling wheel 12, may float due to the rough roadsurface, the load exerting on the hydraulic motor 14 becoming quitesmall, almost all amount of the high pressure fluid discharged from thehydraulic pump 11 is supplied to the hydraulic motor 14, so that thetraveling wheel 12 is skidding to make traveling impossible.

[0005] In this case, by switching the opening-closing valve 19 from anopen to closed state, the flow passing areas of the bifurcate portions16 a, 16 b are controlled by the flow dividing valve 17 in accordancewith the loads applied on the hydraulic motors 14, 15, so that the highpressure fluid from the hydraulic pump 11 is supplied in equal amount tothe hydraulic motors 14, 15 to continue the traveling.

[0006] In the related hydraulic circuit, the flow dividing valve 17 isused to supply an equal amount of high pressure fluid to the hydraulicmotors 14, 15. However, since this flow dividing valve 17 produces agreat pressure loss when the fluid passes through it, the hydraulicmotors 14, 15 have the lower pressure applied. As a result, there is theproblem that the rotational torque (traction force) is lower in thescene where a great traction force is required.

SUMMARY OF THE INVENTION

[0007] It is an object of this invention to provide a hydraulic circuitfor traveling that can rotate two hydraulic motors with a greatrotational torque and at almost equal speed, even if different loads areapplied on both hydraulic motors.

[0008] In order to solve the aforesaid object, the invention ischaracterized by having the following arrangement.

[0009] (1) A hydraulic circuit for traveling comprising:

[0010] a hydraulic pump;

[0011] two hydraulic motors for driving and rotating the travelingwheels, respectively;

[0012] a pumping passage for supplying a high pressure fluid dischargedfrom the hydraulic pump to the two hydraulic motors, the pumping passageincluding one end connected to said hydraulic pump and bifurcatedhalfway to provide the other ends being connected to the hydraulicmotors, respectively;

[0013] two rotation detectors for detecting the rotational speeds of thehydraulic motors, respectively;

[0014] a detection controller for detecting whether or not any travelingwheel is skidding by comparing the rotational speeds detected by therotation detectors; and

[0015] two braking force applying members for applying a braking forceonly to the hydraulic motor of the skidding traveling wheel on the basisof the result of detection from the detection controller, the brakingforce applying members being installed in each of the hydraulic motors.

[0016] (2) The hydraulic circuit according to (1), wherein the brakingforce applying member comprises a piston for applying a braking force tothe hydraulic motors when a fluid is supplied, and a control valve forcontrolling the fluid conducted to the piston on the basis of a signalfrom the detection controller.

[0017] (3) The hydraulic circuit according to (2), wherein

[0018] the braking force applying member comprises a friction plate typenegative brake having a rotational friction plate connected to arotational portion of each hydraulic motor, and a stationary counterpartplate, connected to a stationary portion of each hydraulic motor, forapplying a braking force to the rotational portion of said hydraulicmotor when the stationary counterpart plate is brought into frictionallycontact with the rotational friction plate, and

[0019] the rotational friction plate and the stationary counterpartplate are brought into frictional contact with each other by themovement of the piston.

[0020] (4) The hydraulic circuit according to (2), wherein

[0021] the control valve comprises a pressure reducing valve,

[0022] the detection controller is connected to a foot brake, and

[0023] secondary pressure of the pressure reducing valve is increased ordecreased in accordance with amount of treading the foot brake under thecontrol of the detection controller.

[0024] (5) The hydraulic circuit according to (1), wherein

[0025] the detection controller is connected to a steering system forinputting a steering angle of the steering system into the detectioncontroller, and

[0026] when the traveling wheel is steered by an operation of thesteering system, the detection controller does not judge that thetraveling wheel is skidding even if there is a difference in rotationalspeed between said both hydraulic motors due to turning.

[0027] (6) A traction control system for a traveling vehicle having atleast two traveling wheels driven by a single hydraulic pump throughrespective hydraulic motors, comprising:

[0028] rotation detectors for respectively detecting rotation speeds ofthe traveling wheels;

[0029] a computer, connected to the rotation detectors, for comparingthe rotational speeds of the traveling wheels; and

[0030] computer-controlled brake units respectively provided to thetraveling wheels, and independently controlled by the computer.

[0031] (7) The traction control system according to (6), wherein whenthe computer detects that one of the traveling wheels skids, theassociated computer-controlled brake unit applies a braking force to theskidding traveling wheel.

[0032] (8) The traction control system according to (7), wherein

[0033] the computer is connected to a steering system for inputting asteering angle of the steering system into the computer, so as to detectthe skidding traveling wheel based on the detected rotation speeds andthe inputted steering angle.

[0034] (9) The traction control system according to (6), wherein thecomputer is connected to a foot brake, and controls thecomputer-controlled brake units according to an operation of the footbrake.

[0035] (10) The traction control system according to (6) furthercomprising a parking brake for applying braking force to both of thetraveling wheels when the hydraulic pump is not driven or the hydraulicpump does not provide sufficient hydraulic pressure to the hydraulicmotors.

[0036] (11) The traction control system according to (10), wherein thecomputer controls the computer-controlled brake units based on thedifference between the rotation speeds of the traveling wheel.

[0037] (12) A brake system for a traveling vehicle having at least twotraveling wheels driven by a single hydraulic pump through respectivehydraulic motors, comprising:

[0038] a parking brake for applying braking force to both of thetraveling wheels when the hydraulic pump is not driven or the hydraulicpump does not provide sufficient hydraulic pressure to the hydraulicmotors; and

[0039] an auxiliary brake for applying braking force to only one of thetraveling wheels independently of the parking brake.

[0040] (13) The brake system according to (12), wherein

[0041] the parking brake includes first pistons for respectivelyapplying the braking force to both of the traveling wheels when thesufficient hydraulic pressure does not act on the first pistons; and

[0042] the auxiliary brake includes second pistons for the respectivetraveling wheel so that the respective second piston supplies thebraking force to the corresponding traveling wheel when a predeterminedhydraulic pressure acts on the second piston.

[0043] (14) The brake system according to (13), wherein the first pistonand the second piston are formed integrally with each other with respectto each traveling wheel.

[0044] (15) The brake system according to (13), wherein the first pistonand the second are formed individually with each other with respect toeach traveling wheel.

[0045] (16) The brake system according to (13), wherein

[0046] each of the hydraulic motors includes a friction plate connectedto a rotational portion for driving the traveling wheel and a stationarycounterpart plate connected to a stationary portion thereof for applyingthe braking force to the rotational portion when the stationarycounterpart plate is brought into frictionally contact with therotational friction plate.

[0047] (17) The brake system according to (14), wherein

[0048] the rotational friction plate and the stationary counterpartplate are brought into frictional contact with each other by being urgedby the first piston or the second piston.

[0049] (18) The brake system according to (12) further comprising:

[0050] rotation detectors for detecting rotational speeds of thetraveling wheels, respectively; and

[0051] a computer, connected to the rotation detectors, for comparingthe rotational speeds of the traveling wheels,

[0052] wherein the computer controls the auxiliary brake based on aresult of comparison.

[0053] (19) The brake system according to (18), wherein

[0054] when the computer detects that one of the traveling wheels skids,the auxiliary brake unit applies a braking force to the skiddingtraveling wheel.

[0055] For example, suppose that one traveling wheel floats from theroad surface, and one hydraulic motor for driving the traveling wheelhas a quite small load. In such a case, the high pressure fluid from thehydraulic pump is substantially supplied to one hydraulic motor, so thatone hydraulic motor is rotating (skidding) at high speed and the otherhydraulic motor hardly rotates. Herein, the rotational speeds of thehydraulic motors are detected by the rotation detector, respectively,and the results of detection are output to detection controller. At thistime, the detection controller detects that one traveling wheel isskidding by comparing the results of detection.

[0056] In this way, if one traveling wheel is detected to be skidding,the detection controller activates one braking force applying member toapply a braking force to one hydraulic motor alone, and balance the loadof one hydraulic motor with that of the other hydraulic motor. Thereby,the high pressure fluid is also supplied to the other hydraulic motor,which is then rotated to continue the traveling. And if the otherhydraulic motor is rotated, all the fluid discharge from the hydraulicpump is not supplied to one hydraulic motor, whereby it is possible toprevent one hydraulic motor from being damaged due to over speed.

[0057] Since there is no need of employing the flow dividing valve torelease the skidding, the high pressure fluid supplied to both hydraulicmotors produces less pressure loss, so that both hydraulic motors can beeasily rotated with a great rotational torque and at almost equal speed.

[0058] Also, the braking force can be applied to the hydraulic motors ina simple constitution.

[0059] Further, the negative brakes installed in the hydraulic motorscan be directly employed to apply the braking force to the travelingwheels that are skidding.

[0060] Also, the braking force applied to the hydraulic motors can beeasily controlled.

[0061] Moreover, even if there is a difference in rotational speedbetween both hydraulic motors at the time of turning, it is possible toprevent the false recognition that the traveling wheel is skidding.

[0062] The present disclosure relates to the subject matter contained inJapanese patent application No. 2000-302642 (filed on Oct. 2, 2000),which is expressly incorporated herein by reference in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 is a circuit diagram showing one embodiment of the presentinvention.

[0064]FIG. 2 is a cross-sectional view around a negative brake.

[0065]FIG. 3 is a circuit diagram showing one example of the relatedhydraulic circuit for traveling.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0066] One embodiment of the present invention will be described belowwith reference to the accompanying drawings.

[0067] In FIGS. 1 and 2, reference numeral 21 denotes a variabledisplacement hydraulic pump, and this hydraulic pump 21, when driven byan engine 22, discharges a high pressure fluid through a flowinlet-outlet opening 21 a or 21 b. Reference numerals 23, 24 denote twovariable displacement hydraulic motors having the flow inlet-outputopenings 23 a, 23 b and the flow inlet-outlet openings 24 a, 24 b,respectively. A pumping passage 25 includes one end connected to theflow inlet-outlet opening 21 a of the hydraulic pump 21, and isbifurcated halfway to provide other ends respectively connected to theflow inlet-outlet openings 23 a, 24 a of these hydraulic motors 23, 24.A pumping passage 26 includes one end connected to the flow inlet-outletopening 21 b of the hydraulic pump 21 and is bifurcated halfway toprovide other ends respectively connected to the flow inlet-outletopenings 23 b, 24 b of these hydraulic motors 23, 24.

[0068] Consequently, a high pressure fluid discharged from the hydraulicpump 21 is supplied through the pumping passage 25 or 26 to twohydraulic motors 23, 24, and a low pressure fluid flowing from thehydraulic motors 23, 24 is returned through the remaining pumpingpassage 25 or 26 to the hydraulic pump 21, so that the hydraulic motors23, 24 are driven to rotate the rotational shafts 23 c, 24 c in aforward or backward direction. Since the traveling wheels 27, 28 for thecivil construction machine or the like are connected to these rotationalshafts 23 c, 24 c, respectively, these traveling wheels 27, 28 arerotated together with the rotational shafts 23 c, 24 c, whereby thecivil construction machine can travel.

[0069] Herein, the hydraulic motors 23, 24 employs a swash plate typehydraulic motor having a swash plate 31 capable of changing the angle ofinclination thereof, as shown in FIG. 2. Also, one or more ring-likestationary counterpart plates 33 are connected axially movably on astationary portion of the hydraulic motor 23, 24 (the inner peripheriesof the stationary casing 32 of the hydraulic motor 23, 24 in thisembodiment) by spline connection or mating at two positions. On theother hand, one or more ring-like rotational friction plates 35 areconnected axially movably on the rotational portion of the hydraulicmotor 23, 24 (the outer peripheries of a cylinder block 34 of thehydraulic motor 23, 24 in this embodiment) by spline connection ormating at two positions to be rotatable integrally with the cylinderblock 34. The stationary counterpart plate 33 and the rotationalfriction plate 35 are disposed alternately in the axial direction.

[0070] Reference numeral 37 denotes a plurality of springs disposed onone side of the rotational friction plate 35, these springs 37 bring thestationary counterpart plate 33 and the rotational friction plate 35into frictional contact with each other via a substantially cylindricalbraking piston 38, providing a braking force to the rotational portionof the hydraulic motors 23, 24. Reference numeral 39 denotes a cylinderchamber formed between the stationary casing 32 and the braking piston38, in which if a high pressure fluid is supplied into the cylinderchamber 39, the braking piston 38 is moved to one side against thesprings 37. Thereby, the braking piston 38 is separated from thestationary counterpart plate 33 and the rotational friction plate 35 todisengage them from the frictional contact, so as to release the brakingfor the rotational portion.

[0071] On one hand, if a fluid is exhausted from the cylinder chamber39, the braking piston 38 is urged by the springs 37 and brings thestationary counterpart plate 33 and the rotational friction plate 35into frictional contact with each other. The stationary counterpartplate 33, the rotational friction plate 35, the springs 37 and thebraking piston 38, as a whole, constitute a pair of negative brakes 40,41 to provide a braking force to the hydraulic motors 23, 24.

[0072] Reference numeral 45 denotes a hydraulic pump which is drivenwith the hydraulic pump 21 by the engine 22 to discharge a high pressurefluid, a suction opening 45 a of this hydraulic pump 45 being connectedthrough a suction passage 47 to a tank 46. A discharge opening 45 b ofthe hydraulic pump 45 is connected to one end of a supply passage 48,which is bifurcated halfway, the other ends of the supply passage 48being connected to the cylinder chambers 39 of the negative brakes 40,41, respectively. A directional control valve 49 is interposed in thesupply passage 48 between the hydraulic pump 45 and the branch portion.If this directional control valve 49 is switched to an oblique flowposition, a high pressure fluid discharged from the hydraulic pump 45 issupplied to the cylinder chambers 39 of the negative brakes 40, 41. Onthe other hand, if the directional control valve 49 is switched to aparallel flow position, a fluid from the cylinder chambers 39 of thenegative brakes 40, 41 is exhausted into the tank 46.

[0073] Reference numeral 51 denotes a make-up feed passage having oneend connected to the supply passage 48 between the hydraulic pump 45 andthe directional control valve 49, bifurcated halfway, with the otherends connected to the pumping passages 25, 26, respectively. Thebifurcate portions 51 a, 51 b of this make-up feed passage 51 areprovided with the check valves 52, 53 for permitting only a flow offluid to the pumping passages 25, 26, respectively. And a high pressurefluid discharged from the hydraulic pump 45 is refilled through themake-up feed passage 51 to the pumping passage 25 or 26 on the lowerpressure side, when the check valve 52 or 53 is opened.

[0074] Reference numerals 54, 55 denotes a relief valve provided in themake-up feed passage 51, these relief valves 54, 55 being arranged inparallel to the check valves 52, 53, respectively. Reference numeral 56denotes a relief passage having one end connected to the supply passage48 between the hydraulic pump 45 and the directional control valve 49,with the other end being connected to the tank 46, a relief valve 57being interposed halfway in this relief passage 56.

[0075] Reference numeral 59 denotes a flushing valve, which is connectedto the other ends of the first and second passages 60, 61 having oneends connected to the pumping passages 25, 26, respectively. If thisflushing valve 59 is turn open by a pilot pressure from the first orsecond passage 60 or 61 on the high pressure side, a fluid in the firstor second passage 60, 61 on the low pressure side is flowed out througha flow-out passage 62 to a drain chamber of the hydraulic pump 21,refreshing the fluid within the pumping passages 25, 26 continuously andcooling the hydraulic pump 21. Reference numeral 63 denotes a lowpressure relief valve interposed halfway in the flow-out passage 62.

[0076] Reference numerals 66, 67 denote the rotation detectors such asrotary encoder for detecting the rotational speed of the hydraulicmotors 23, 24. These two rotation detectors 66, 67 detect the rotationalspeed of the traveling wheels 27, 28 or the rotational shafts 23 c, 24c, thereby detecting the rotational speed of the hydraulic motors 23,24, and output its detection result to detection controller 68 connectedto the rotation detectors 66, 67 and composed of a CPU. The detectioncontroller 68 compares the rotational speeds of the hydraulic motors 23,24 detected by the rotation detectors 66, 67, and determines that eitherthe traveling wheel 27 or 28 is skidding if the difference is greaterthan or equal to a predetermined value.

[0077] Reference numeral 71 denotes a cylindrical piston disposed in asuper posed state radially inside the braking piston 38 constituting thenegative brakes 40, 41. This cylindrical piston 71 is moved to the otherside and brings the stationary counterpart plate 33 and the rotationalfriction plate 35 into frictional contact with each other to provide abraking force to the hydraulic motors 23, 24, if a high pressure fluidis supplied to a cylinder chamber 72 formed between the stationarycasing 32 and the piston 71.

[0078] On the other hand, if the supply of high pressure fluid to thecylinder chamber 72 is stopped, the piston 71 is separated from thestationary counterpart plate 33 and the rotational friction plate 35 todisengage them from the frictional contact, so that the rotationalportion of the hydraulic motors 23, 24 is released from braking. In thisway, the stationary counterpart plate 33 and the rotational frictionplate 35 of the negative brakes 40, 41 are brought into frictionalcontact by the piston 71, the negative brakes 40, 41 already installedin the hydraulic motors 23, 24 can be directly employed to provide thebraking force to the traveling wheels 27, 28 that are skidding,resulting in simple structure and moderate price.

[0079] Reference numeral 74 denotes a fluid passage having one endconnected to the supply passage 48 between the hydraulic pump 45 and thedirectional control valve 49, bifurcated halfway, with the other endsbeing connected to the cylinder chambers 72 of the negative brakes 40,41. In the bifurcate portion 74 a, 74 b of this fluid passage, thecontrol valves (the pressure reducing valves 75, 76 in this embodiment)for controlling the high pressure fluid to be led to the piston 71 inaccordance with a control signal from the detection controller 68 areinterposed.

[0080] The detection controller 68 controls the pressure reducing valve75 or 76 by outputting a control signal to the coil of correspondingpressure reducing valve 75 or 76 if detecting that either of thetraveling wheels 27, 28 is skidding. Thereby, a high pressure fluid issupplied from the hydraulic pump 45 to the cylinder chamber 72 of thehydraulic motor 23 or 24 to provide a braking force to the hydraulicmotor 23 or 24.

[0081] The piston 71 and the pressure reducing valves 75, 76, as awhole, are installed in the hydraulic motors 23, 24, respectively, andconstitute two braking force applying members 77, 78 for applying abraking force only to the skidding hydraulic motor 23, 24 of thetraveling wheel 27, 28 in accordance with the detection result of thedetection controller 68. If each braking force applying members 77, 78is constituted by the piston 71 and the pressure reducing valves 75, 76,it is possible to apply a braking force to the hydraulic motors 23, 24in a simple construction.

[0082] In this embodiment, the detection controller 68 is connected to afoot brake 80 of the civil construction machine, more particularly, adetection sensor for detecting the amount of treading the foot brake 80,the amount of treading the foot brake 80 detected by the detectionsensor 81 is input into the detection controller 68, and a controlsignal from the detection controller 68 in accordance with the amount oftreading the foot brake 80 is output to the coil (particularly aproportional coil) of the pressure reducing valve 75 or 76, increasingor decreasing the secondary pressure of the pressure reducing valve 75or 76.

[0083] Thereby, the pressure of the fluid supplied to the cylinderchamber 72 of the hydraulic motor 23 or 24, in other words, a brakingforce provided from the negative brakes 40, 41 to the hydraulic motors23, 24 can be easily controlled in a range of appropriate values.

[0084] Herein, if the civil construction machine turns while traveling,the rotational speed of the inward traveling wheel is slower than therotational speed of the outward traveling wheel, thereby bringing aboutthe danger that the detection controller 68 detects the outwardtraveling wheel to be skidding even if the traveling wheel is notactually skidding.

[0085] Therefore, in this embodiment, the detection controller 68 isconnected to the steering system 83, for example, the detection sensor84 for detecting the steering angle (motion amount) of a steering shaft,to input the steering angle of the steering system 83 detected by thedetection sensor 84 into the detection controller 68. Thereby, when thecivil construction machine is turning by the operation of the steeringsystem 83, even though there is a difference in rotational speed betweenthe hydraulic motors 23, 24 depending on the steering angle, it is notdetermined on the basis of such difference that either of the hydraulicmotors 23, 24 is skidding to prevent the false recognition.

[0086] The operation of this embodiment of this invention will be setforth below.

[0087] When the engine 22 is started, the hydraulic pumps 21, 45 arerotated in the forward direction, and a high pressure fluid isdischarged from the hydraulic pumps 21, 45 to the pumping passage 25 andthe supply passage 48, the high pressure fluid discharged from thehydraulic pump 21 is flowed into the hydraulic motors 23, 24simultaneously. Thereby, the rotational shafts 23 c, 24 c of thehydraulic motors 23, 24 are rotated together with the traveling wheels27, 28, so that the civil construction machine moves in forwarddirection. In this time, a return fluid with low pressure that flows outof the hydraulic motors 23, 24 gets back through the pumping passage 26to the hydraulic pump 21, but if the fluid is circulating between thehydraulic pump 21 and the hydraulic motors 23, 24 without returning tothe tank, there is the danger that the temperature rises, causing thedeterioration with time.

[0088] However, in this embodiment, the high pressure fluid dischargedfrom the fluid pomp 45 pushes open the check valve 53 to be alwaysrefilled to the pumping passage 26 on the low pressure side, so that thefluid within the circuit is replaced successively, thereby preventingthe deterioration. And if the fluid is refilled to the pumping passage26, the amount of fluid within the circuit increases to raise thecircuit pressure, but at this time, excess fluid is exhausted throughthe flushing valve 59 and the relief valve 63 to a drain chamber of thehydraulic pump 21. Thereby, tne circuit pressure is prevented fromrising. And the hydraulic pump 21 is cooled.

[0089] At this time, since the directional control valve 49 is switchedto the oblique flow position, the high pressure fluid discharged fromthe hydraulic pump 45 is supplied through the supply passage 48 to thecylinder chamber 39 of the negative brakes 40, 41. As a result, thebraking pistons 38 of the negative brakes 40, 41 are separated from thestationary counterpart plate 33 and the rotational friction plate 35 todisengage them from the frictional contact, and release the rotationalportion of the hydraulic motors 23, 24 from braking.

[0090] When the machine moves in the forward direction, the rotationalspeeds of the traveling wheels 27, 28 detected by the rotation detectors66, 67 are output to the detection controller 68. However, since therotational speeds of the traveling wheels 27, 28 are substantially thesame, even if both rotational speeds are compared, its difference isless than the predetermined value. Consequently, the detectioncontroller 68 determines that any traveling wheels 27, 28 are notskidding. In such case, the detection controller 68 controls thepressure reducing valves 75, 76 so that the secondary pressure of thepressure reducing valves 75, 76 becomes zero by outputting a controlsignal to the coil of the pressure reducing valves 75, 76. Thereby, nohigh pressure fluid is supplied to the cylinder chamber 72 of thenegative brakes 40, 41, so that no braking force is provided to therotational portion of any of the hydraulic motors 23, 24.

[0091] For example, suppose that the traveling wheel 27 floats from theroad surface, and the load of the hydraulic motor 23 for driving thetraveling wheel 27 is quite small. At such time, since almost all thehigh pressure fluid from the hydraulic pump 21 is supplied to thehydraulic motor 23, the hydraulic motor 23 is rotated (skidding) at highrate, bringing about the danger that the hydraulic motor 24 can behardly rotated.

[0092] However, in this embodiment, the hydraulic motor 23 is releasedfrom skidding, and the hydraulic motors 23, 24 are rotated at theapproximately same rotational speed, as described below. That is, therotational speeds of the hydraulic motors 23, 24 are detected by therotation detectors 66, 67, and the detection result are output to thedetection controller in the same manner as described previously. At thistime, the detection controller 68 compares the detection results, sincethe differential value from the rotational speed of the hydraulic motor23 subtracted by the rotational speed of the hydraulic motor 24 isgreater than or equal to the predetermined value, the traveling wheel 23is detected to be skidding.

[0093] In this way, if the traveling wheel 23 is detected to beskidding, the detection controller 68 outputs a control signal inaccordance with the amount of treading the foot brake 80 to the coil(proportional coil) of corresponding pressure reducing valve 75, andincreases the secondary pressure of the pressure reducing valve 75 inaccordance with the value of the control signal. Thereby, the fluid atan appropriate pressure is supplied to the cylinder chamber 72 of thehydraulic motor 23, so that the piston 71 is moved to the other side. Asa result, the stationary counterpart plate 33 and the rotationalfriction plate 35 for the negative brake 40 are brought into frictionalcontact with each other to provide a braking force of appropriate valueonly to the hydraulic motor 23.

[0094] Consequently, the load exerting on the hydraulic motor 23approaches to the load on the hydraulic motor 24, so that the highpressure fluid from the hydraulic pump 21 is supplied to the hydraulicmotor 24. Thereby, the hydraulic motor 24 and the traveling wheel 28 arerotated, whereby the civil construction machine can travel continuously.And if the hydraulic motor 24 is rotated, as described previously, allthe fluid discharged from the hydraulic pump 21 is not supplied to thehydraulic motor 23, thereby preventing the hydraulic motor 23 from beingdamaged due to over speed.

[0095] Since there is no need of employing the flow dividing valve torelease the skidding, the high pressure fluid supplied to the hydraulicmotors 23, 24 does not have pressure loss, so that both hydraulic motors23, 24 can be easily rotated with large rotational torque and at almostequal speed.

[0096] In the above embodiment, a piston 71 different from the brakingpiston 38 for the negative brakes 40, 41 is provided, and the stationarycounterpart plate 33 and the rotational friction plate 35 for thenegative brakes 40, 41 are brought into frictional contact with eachother by the piston 71. However, the fluid from the control valve may beled to the braking piston of the negative brake, to bring the stationarycounterpart plate and the rotational friction plate into frictionalcontact with each other.

[0097] In the above embodiment, the pressure of the fluid led to thepiston 71 of the braking force applying members 77, 78 is adjusted bychanging the secondary pressure of the pressure reducing valves 75, 76in accordance with the amount of treading the foot brake 80. However, inthis invention, the control valve may be a simple opening-closing valveso that the fluid at equal pressure to that within the fluid passage isled to the piston of braking force applying members.

[0098] Further, in the above embodiment, the braking piston 38 and thepiston 71 are separately provided. However, the braking piston 38 andthe piston 71 may be formed integrally so as to form an integral piston.In this case, the cylinder chamber 39 and the cylinder chamber 72 arerespectively formed at the opposite sides of the integral piston so thatthe braking force is applied when the high pressure fluid is notsupplied into the cylinder chamber 39 or when the high pressure fluid issupplied into the cylinder chamber 72, and so that the braking force isnot applied when the high pressure fluid is supplied into the cylinderchamber 39 or when the high pressure fluid is not supplied into thecylinder chamber 72. That is, when the sum of the spring force of thespring 37 and the pressure in the cylinder chamber 72 is larger than thepressure in the cylinder chamber 39, the braking force is applied, andwhen the sum of the spring force of the spring 37 and the pressure inthe cylinder chamber 72 is smaller than the pressure in the cylinderchamber 39, the braking force is not applied. Consequently, theconstruction of the cylinder is simplified.

[0099] Further, in the above embodiment, the piston 38 and the piston 71act on the same stationary counterpart plate 33 and the rotationalfriction plate 35 to generate the braking force. However, an individualset off the stationary counterpart plate and the rotational frictionplate may be provided, each being driven by a respective piston 38 or71.

[0100] In the above embodiment, the present invention has been describedwith reference to the construction of the hydraulic circuit. The presentinvention is also featured by a traction control system for thetraveling vehicle which includes: rotation detectors for respectivelydetecting rotation speeds of the traveling wheels 27, 28; a computer,connected to the rotation detectors, for comparing the rotational speedsof the traveling wheels; and computer-controlled brake unitsrespectively provided to the traveling wheels, and independentlycontrolled by the computer.

[0101] The rotation detectors correspond to the rotation detectors 66,67, the computer corresponds to the detection controller 68, and thecomputer-controlled brake units correspond to the braking force applyingmembers 77, 78 and the pressure reducing valves 75, 76 in the aboveembodiment.

[0102] Since the computer is connected to the rotation detectors and iscapable of processing the outputs from the rotation detectors andindividually controlling the computer-controlled brake units, thecomputer-controlled brake units can be suitably controlled by thecomputer. For example, when the computer detects, based on the outputfrom the rotation detectors, that the one of traveling wheels isskidding, the computer outputs a control signal to thecomputer-controlled brake unit, with which the skidding traveling wheelis associated, for applying a braking force to the skidding travelingwheel. Therefore, the skidding traveling wheel stops skidding and all ofthe traveling wheels are normally rotated.

[0103] The computer may be further connected to the foot brake and thesteering system. In this case, the computer can control thecomputer-controlled brake units based on the outputs from the rotationdetectors and further the foot brake and the steering system asmentioned above. Therefore, braking controllability of the computer isenhanced.

[0104] Further, a parking brake corresponding to the negative brakes 40,41 in the above embodiment may be employed in the traction controlsystem. The parking brake is adapted to apply the braking force to allof the traveling wheels when the hydraulic pump is not driven or thehydraulic pump does not provide sufficient hydraulic pump to thehydraulic motors and is separately controlled from thecomputer-controlled brake units. Therefore, the traveling wheels can becontrolled by both of the computer-controlled brake units and theparking brake depending on the purpose of brake.

[0105] The present invention is also featured by a brake system for thetraveling vehicle includes: a parking brake for applying braking forceto the traveling wheels 27, 28 when the hydraulic pump is not driven orthe hydraulic pump does not provide sufficient hydraulic pressure to thehydraulic motors; and an auxiliary brake for applying braking force toonly one of the traveling wheels independently of the parking brake.

[0106] The parking brake corresponds to the negative brakes 40, 41, andthe auxiliary brake corresponds to the braking force applying members77, 78 and the pressure reducing valves 75, 76 in the above embodiment.

[0107] The parking brake is adapted to apply the braking force to thetraveling wheels 27, 28 so that the braking force for the travelingwheels 27, 28 is substantially same with each other, and the auxiliarybrake is adapted to apply the braking force only one of the travelingwheels. Therefore, the suitable braking force for each traveling wheelcan be applied by combining the parking brake and the auxiliary brake.Particularly, when one of the traveling wheels is skidding, theauxiliary brake applies the braking force to the skidding travelingwheel, so that the skidding traveling wheel stops skidding and all ofthe traveling wheels can be normally rotated.

[0108] A computer corresponding to the detection controller 68 in theabove embodiment may be connected to the auxiliary brake to control it.The computer is connected to the rotation detectors 66, 67 for detectingrotational speeds of the traveling wheels, so that the computer cancontrol the auxiliary brake based on the rotational speeds of thetraveling wheels.

[0109] As described above, with this invention, even if two hydraulicmotors have different loads, the hydraulic motors can be rotated with alarge rotational torque and at almost equal speed.

What is claimed is
 1. A hydraulic circuit for traveling comprising: ahydraulic pump; two hydraulic motors for driving and rotating thetraveling wheels, respectively; a pumping passage for supplying a highpressure fluid discharged from the hydraulic pump to the two hydraulicmotors, the pumping passage including one end connected to saidhydraulic pump and bifurcated halfway to provide the other ends beingconnected to the hydraulic motors, respectively; two rotation detectorsfor detecting the rotational speeds of the hydraulic motors,respectively; a detection controller for detecting whether or not anytraveling wheel is skidding by comparing the rotational speeds detectedby the rotation detectors; and two braking force applying members forapplying a braking force only to the hydraulic motor of the skiddingtraveling wheel on the basis of the result of detection from thedetection controller, the braking force applying members being installedin each of the hydraulic motors.
 2. The hydraulic circuit according toclaim 1, wherein the braking force applying member comprises a pistonfor applying a braking force to the hydraulic motors when a fluid issupplied, and a control valve for controlling the fluid conducted to thepiston on the basis of a signal from the detection controller.
 3. Thehydraulic circuit according to claim 2, wherein the braking forceapplying member comprises a friction plate type negative brake having arotational friction plate connected to a rotational portion of eachhydraulic motor, and a stationary counterpart plate, connected to astationary portion of each hydraulic motor, for applying a braking forceto the rotational portion of said hydraulic motor when the stationarycounterpart plate is brought into frictionally contact with therotational friction plate, and the rotational friction plate and thestationary counterpart plate are brought into frictional contact witheach other by the movement of the piston.
 4. The hydraulic circuitaccording to claim 2, wherein the control valve comprises a pressurereducing valve, the detection controller is connected to a foot brake,and secondary pressure of the pressure reducing valve is increased ordecreased in accordance with amount of treading the foot brake under thecontrol of the detection controller.
 5. The hydraulic circuit accordingto claim 1, wherein the detection controller is connected to a steeringsystem for inputting a steering angle of the steering system into thedetection controller, and when the traveling wheel is steered by anoperation of the steering system, the detection controller does notjudge that the traveling wheel is skidding even if there is a differencein rotational speed between said both hydraulic motors due to turning.6. A traction control system for a traveling vehicle having at least twotraveling wheels driven by a single hydraulic pump through respectivehydraulic motors, comprising: rotation detectors for respectivelydetecting rotation speeds of the traveling wheels; a computer, connectedto the rotation detectors, for comparing the rotational speeds of thetraveling wheels; and computer-controlled brake units respectivelyprovided to the traveling wheels, and independently controlled by thecomputer.
 7. The traction control system according to claim 6, whereinwhen the computer detects that one of the traveling wheels skids, theassociated computer-controlled brake unit applies a braking force to theskidding traveling wheel.
 8. The traction control system according toclaim 7, wherein the computer is connected to a steering system forinputting a steering angle of the steering system into the computer, soas to detect the skidding traveling wheel based on the detected rotationspeeds and the inputted steering angle.
 9. The traction control systemaccording to claim 6, wherein the computer is connected to a foot brake,and controls the computer-controlled brake units according to anoperation of the foot brake.
 10. The traction control system accordingto claim 6 further comprising a parking brake for applying braking forceto both of the traveling wheels when the hydraulic pump is not driven orthe hydraulic pump does not provide sufficient hydraulic pressure to thehydraulic motors.
 11. The traction control system according to claim 10,wherein the computer controls the computer-controlled brake units basedon the difference between the rotation speeds of the traveling wheel.12. A brake system for a traveling vehicle having at least two travelingwheels driven by a single hydraulic pump through respective hydraulicmotors, comprising: a parking brake for applying braking force to bothof the traveling wheels when the hydraulic pump is not driven or thehydraulic pump does not provide sufficient hydraulic pressure to thehydraulic motors; and an auxiliary brake for applying braking force toonly one of the traveling wheels independently of the parking brake. 13.The brake system according to claim 12, wherein the parking brakeincludes first pistons for respectively applying the braking force toboth of the traveling wheels when the sufficient hydraulic pressure doesnot act on the first pistons; and the auxiliary brake includes secondpistons for the respective traveling wheel so that the respective secondpiston supplies the braking force to the corresponding traveling wheelwhen a predetermined hydraulic pressure acts on the second piston. 14.The brake system according to claim 13, wherein the first piston and thesecond piston are formed integrally with each other with respect to eachtraveling wheel.
 15. The brake system according to claim 13, wherein thefirst piston and the second are formed individually with each other withrespect to each traveling wheel.
 16. The brake system according to claim13, wherein each of the hydraulic motors includes a friction plateconnected to a rotational portion for driving the traveling wheel and astationary counterpart plate connected to a stationary portion thereoffor applying the braking force to the rotational portion when thestationary counterpart plate is brought into frictionally contact withthe rotational friction plate.
 17. The brake system according to claim14, wherein the rotational friction plate and the stationary counterpartplate are brought into frictional contact with each other by being urgedby the first piston or the second piston.
 18. The brake system accordingto claim 12 further comprising: rotation detectors for detectingrotational speeds of the traveling wheels, respectively; and a computer,connected to the rotation detectors, for comparing the rotational speedsof the traveling wheels, wherein the computer controls the auxiliarybrake based on a result of comparison.
 19. The brake system according toclaim 18, wherein when the computer detects that one of the travelingwheels skids, the auxiliary brake unit applies a braking force to theskidding traveling wheel.